Process parameters for hot stamping of AA7075 and D-7xxx to achieve high performance aged products

Abstract

This work examines the necessary process parameters for die quenching (DQ) during hot stamping and subsequent age hardening and paint bake cycle (PBC) response for two alloys: AA7075 and a developmental 7xxx alloy (referred to as AA7xxx), with a lower Chromium content, higher Zirconium content and higher Zinc-to-Magnesium ratio in comparison to AA7075. For both alloys, a minimum solutionizing time of 8?min was found to be required, along with a minimum quench rate of 56?°C/s and 27?°C/s for AA7075 and AA7xxx, respectively. Two-step aging treatments, leveraging a paint bake cycle (PBC) of 177?°C for 30?min as the second step, were considered after die quenching and were devised to achieve T6- or T76-level strengths. For AA7075, an aging treatment of 120?°C for 8?h, followed by the paint bake cycle (PBC) produced strength levels similar to a T6 temper. DSC experiments showed that the microstructure from this heat treatment was similar to a peak-aged T6 temper. For AA7xxx, a treatment of 100?°C for 4?h and followed by the paint bake produced a strength similar to a T76 temper, while 120?°C for 3?h followed by the paint bake yielded T6 strength levels. The properties of the custom aging treatments were validated through tensile tests. The resulting stress-strain curves show that it is possible to achieve T6 or T76 properties using a custom aging treatment incorporating the PBC that is 65–83% shorter than standard T6 or T76 treatments.